DC/AC converter and method of controlling a DC/AC converter

ABSTRACT

A DC/AC converter includes a DC/DC conversion stage with galvanic isolation and a DC/AC conversion stage, wherein the DC/DC conversion stage comprises a pair of first side terminals providing or receiving a first DC voltage, a pair of second side terminals providing or receiving a second DC voltage and coupled to the DC/AC conversion stage, at least one first side converter circuit coupled between the pair of first side terminals, a series connection of a plurality of second side converter circuits coupled between the pair of second side terminals, and at least one transformer circuit coupling the plurality of second side converter circuits to the at least one first side converter circuit, wherein a connection point between two of the plurality of second side converter circuits is coupled to the DC/AC conversion stage and forms a neutral phase point thereof.

RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §371 ofInternational Patent Application No. PCT/EP/2011/059516, having aninternal filing date of Jun. 8, 2011, the content of which isincorporated herein by reference in its entirety.

FIELD

The present invention is directed to a DC/AC converter and to a methodof controlling a DC/AC converter.

BACKGROUND

DC/AC converters are in widespread use. A typical field of use for DC/ACconverters, in particular high power DC/AC converters, are isolatednetworks, which are operated at AC voltages and whose power supply is aDC power source.

The high level structure of a common DC/AC converter 2 is shown inFIG. 1. The DC/AC converter 2 comprises a DC/DC conversion stage 4 and aDC/AC conversion stage 6. The DC/DC conversion stage 4 allows for avoltage conversion between a first DC voltage V_(DC,Prim) and a secondDC voltage V_(DC,Sec). The DC/AC conversion stage 6 allows for a voltageconversion between the second DC voltage V_(DC,Sec) and an AC voltageV_(AC). While the DC/AC conversion stage 6 is a necessary portion of theDC/AC converter 2, the provision of the DC/DC conversion stage isdependent on the particular requirements for the DC/AC converter 2. Sucha DC/DC conversion stage 4 may be provided, because it allows for agalvanic isolation between the DC end and the AC end of the DC/ACconverter. It also allows for the provision of a DC voltage to the DC/ACconversion stage 6 that may be different from V_(DC,Prim) supplied by aDC power source.

FIG. 2 shows a previous approach DC/AC conversion stage 6 having threeAC phases. A DC voltage, in the present case denoted V_(DC,Sec), ispresent at the DC side of the DC/AC conversion stage 6. At its AC side,the DC/AC conversion stage 6 has three AC terminals 80, 82 and 84, eachof which is associated with one of the three phases of the AC voltageV_(AC). Besides, the DC/AC conversion stage 6 has a neutral terminal 86,which provides the voltage reference, also denoted neutral phase, to thethree AC terminals 80, 82 and 84. The neutral terminal is connected toground. The neutral terminal 86 may be the center point of an AC starconfiguration having the three AC phases present at the AC terminals 80,82 and 84. The DC/AC conversion stage 6 further comprises four halfbridge converters 70, 72, 74 and 76, all of which are coupled to the DCside terminals of the DC/AC conversion stage 6. Three of these halfbridge converters, namely the converters 70, 72 and 74, are associatedwith the three phases of the AC voltage, while the fourth converter 76is associated with the neutral phase of the AC voltage system. Thesefour half bridge converters are coupled to the three AC terminals 80, 82and 84 and to the neutral terminal 86 via a filter 78, wherein thefilter 78 is provided for conditioning the AC voltage system.

FIG. 3 shows a previous approach implementation of the DC/DC conversionstage 4 having a pair of first side terminals 10, across which the DCvoltage V_(DC,Prim) is coupled, and a pair of second side terminals 12,across which the DC voltage V_(DC,Sec) is coupled. The pair of firstside terminals are coupled to a first side converter circuit 20, whichin turn is coupled to a transformer circuit 140, which in turn incoupled to a second side converter circuit 130, which in turn is coupledto the second side terminals 12. Each of the first side convertercircuit 20 and the second side converter circuit 130 comprises an Hbridge circuit, whose switches are controlled in such a way that adesired power transfer from the first side terminals to the second sideterminals or from the second side terminals to the first side terminalstakes place.

Previous approach DC/AC converters, as described above, have thedisadvantage that they deviate from their desired electric behaviour inan unacceptable manner when changes in the operating conditions occur.Such changes may consist of varying electric loads applied to the DCside or to the AC side. Such changes can also consist of a change ofpower flow direction between the DC side and the AC side. Thesedeviations from the desired behaviour can occur both in stationarystates as well as relate to the dynamic response of the DC/AC converterto operating condition gradients.

Therefore, the problem underlying the present invention is to provide aDC/AC converter and a method of controlling a DC/AC converter that allowfor a reduced sensitivity of the conversion system to changes inoperating conditions, i.e. to allow an acceptable system behaviour overa wider range of operating conditions.

SUMMARY

This problem is solved by the DC/AC converter in accordance with theclaims.

The claimed DC/AC converter comprises a DC/DC conversion stage withgalvanic isolation and a DC/AC conversion stage, wherein the DC/DCconversion stage comprises a pair of first side terminals providing orreceiving a first DC voltage, a pair of second side terminals providingor receiving a second DC voltage and coupled to the DC/AC conversionstage, at least one first side converter circuit coupled between thepair of first side terminals, a series connection of a plurality ofsecond side converter circuits coupled between the pair of second sideterminals, and at least one transformer circuit coupling the pluralityof second side converter circuits to the at least one first sideconverter circuit, wherein a connection point between two of theplurality of second side converter circuits is coupled to the DC/ACconversion stage and forms a neutral phase point thereof.

By providing a neutral phase point for the DC/AC conversion stage withinthe DC/AC converter, in particular within the DC/DC conversion stage,the need for a separate voltage reference for the AC voltage of theDC/AC converter is eliminated. This is particularly advantageous, asproviding the separate voltage reference required, in previousapproaches as described above, the provision of a separate half bridgeconverter between the pair of second side terminals. The switching ofsaid separate half bridge converter introduced undesired signalcomponents, such that an imperfect reference point was present. As theundesired signal components varied with different operating conditions,for example with different loads at the AC terminals, the accuracy ofthe voltage reference point, and therewith the accuracy of the ACvoltages, could not be ensured over a wide range of operatingconditions. Accordingly, eliminating this separate half bridge converterallows for eliminating the signal artifacts introduced by the switchingthereof. A highly reliable reference point is provided.

The proposed structure of the DC/AC converter has additional advantages.The elimination of the separate half bridge for the voltage referenceallows for a reduction of circuit components, therefore reducing spaceand power requirements as well as cost of the DC/AC converter. Moreover,the provision of a plurality of second side converter circuits allowsfor partitioning the power to be transferred through the DC/DCconversion stage among several power transfer paths, such that each ofthe second side converter circuits can be dimensioned for a lowermaximum power transfer capability than previous approach system havingonly one second side converter circuit. Accordingly, cheaper and/orsmaller components may be used for the second side converter circuits.

Furthermore, the series connection of the second side converter circuitsconveniently allows for providing a connection point between two of thesecond side converter circuits, which is used as the neutral phasepoint. The series connection provides an elegant way of creating theneutral phase point without further circuit components. The DC/DCconversion stage may in operation be controlled in such a way that thevoltages between the connection point, which is the neutral phase point,and each of the pair of second side terminals are equal in magnitude. Inthis way, the voltages at the AC terminals of the DC/AC controller canswing around the voltage at the neutral phase point to equal positiveand negative magnitudes. However, it is also possible that the voltageat the connection point is controlled to have a value different from themid-point between the voltages at the pair of second side terminals,with an additional voltage source being coupled between the connectionpoint and the neutral terminal of the DC/AC conversion stage forensuring a neutral voltage at the neutral terminal. In either case, thevoltage at the connection point may be controlled to have a presetvalue, such that it forms the reference point for the DC/AC conversionstage. It is also possible that the voltage at the connection point iscontrolled to result in unsymmetrical values of the voltages at the pairof second side terminals, while achieving the same voltage differencebetween the AC terminals and the neutral phase point with running thehalf bridge converters with less duty cycle.

The pair of first side terminals form a DC end of the DC/AC converterand may therefore also be referred to as the external connectionterminals of the DC/DC conversion stage, whereas the pair of second sideterminals are coupled to the DC/AC conversion stage and may therefore bereferred to as the internal connection terminals of the DC/DC conversionstage. The language providing or receiving a DC voltage is used toindicate that the DC/AC converter is not necessarily restricted to onepower transfer direction. The claimed structure of the DC/AC converterallows for power transfer from the first side terminals to the ACterminals as well as from the AC terminals to the first side terminals,wherein the power transfer direction may be switched. Accordingly, inthe DC/DC conversion stage, each of the pair of first side terminals andthe pair of second side terminals may be the source or the sink of apower transfer. In certain usage scenarios, however, it is possible thatpower transfer is carried out in one direction only.

According to a further embodiment of the invention, the seriesconnection of the plurality of second side converter circuits consistsof an even number of second side converter circuits and said connectionpoint is a center point of the series connection of the plurality ofsecond side converter circuits. In other words, there may be provided aneven number M of second side converter circuits, with one set of M/2second side converter circuits being coupled between one of the secondside terminals and the center point and another set of M/2 second sideconverter circuits being coupled between the other of the second sideterminals and the center point. In this way, when the voltages acrossthe respective terminals of the second side converter circuits arecontrolled to be equal, the center point assumes a voltage value that ishalf-way between the voltages at the pair of second side terminals.Therefore, the center point is well-suited for forming a neutral phasepoint of the DC/AC conversion stage. In an alternative embodiment,different numbers of second side converter circuits may be coupled onthe two side of said connection point. In this case, the voltages attheir respective terminals may be controlled to have differentmagnitudes, such that the connection point still forms a center point ofthe DC voltage across the second side terminals in an electric sense.

According to a further embodiment, the DC/DC conversion stage hassymmetric power transfer characteristics between the pair of first sideterminals and the pair of second side terminals. In other words, theDC/DC conversion stage exhibits electrically identical behavior for thetwo power transfer directions, namely for the power transfer from thefirst side terminals to the second side terminals and from the secondside terminals to the first side terminals. In this way, it is ensuredthat the DC/DC conversion stage's desired behavior is present in bothpower transfer directions and that the DC/AC converter has superiorbi-directional power transfer properties.

According to a further embodiment, the number of transformer circuits isequal to the number of second side converter circuits, with each one ofthe plurality of second side converter circuits being associated withone of the at least one first side converter circuit via couplingthrough a respective transformer circuit. In particular, each of thesecond side converter circuits is associated with exactly one of the atleast one first side converter circuits. In the case of power transferfrom the second side terminals to the first side terminals, the power tobe transferred is split up between the plurality of second sideconverter circuits. With the association of the second side convertercircuits with the first side converter circuits, it is pre-determinedwhich first side converter circuit will receive the power from a givensecond side converter circuit. Conversely, it is possible that the powerto be transferred from a given first side converter circuit to thesecond side terminals is split up between a plurality of transformercircuits and therewith between a plurality of second side convertercircuit. A set of a given first side converter circuit and theparticular transformer circuit(s) and second side converter circuit(s)to which the power can be transferred from the given first sideconverter circuit can be referred to as a subsystem of the DC/DCconversion stage. The DC/DC conversion stage can have one or moresubsystems. In case there is only one subsystem, said one subsystem hasa plurality of second side converter circuits. In this way, the desiredpower transfer capacity of the DC/DC conversion stage may be dividedamong a plurality of subsystems, such that each subsystem may bedesigned for lower power transfer requirements and may therefore becheaper and/or smaller.

It is also possible that a single transformer circuit couples one firstside converter circuit to a plurality of second side converter circuits.The coupling may be done via a transformer having one set of windings onthe side of the first side converter circuit and a plurality of sets ofwindings, corresponding in number to the plurality of second sideconverter circuits, on the side of the second side converter circuits.

According to a further embodiment, each of the at least one transformercircuit is configured to have a current source characteristic. Thecurrent source characteristics of the transformer circuits are anelegant means for facilitating the symmetric power transfercharacteristics of the DC/DC conversion stage, because they allow thefirst side converter circuits and the second side converter circuits tohave voltage source characteristics, such that the first and second sideconverter circuits can be designed in an identical manner giving rise tothe symmetry of the DC/DC conversion stage. In particular, eachtransformer circuit may comprise a transformer and an inductance elementcoupled in series with the transformer. The inductance element is acircuit element separate from the transformer. However, in analternative embodiment, a parasitic inductance of another circuitelement present in the DC/DC conversion stage may be made use of forestablishing the current source characteristic.

According to a further embodiment, the at least one first side convertercircuit has a voltage source characteristic. If more than one first sideconverter circuit is present, it is possible that the several first sideconverter circuits as a group have a voltage source characteristic. Itis also possible that each first side converter circuit has a voltagesource characteristic. By providing a voltage source characteristic, theat least one first side converter circuit allows for good couplingproperties to a DC source/sink coupled to the pair of first sideterminals.

According to a further embodiment, a first side capacitance element iscoupled between the pair of first side terminals. In this way, onecircuit component, namely the first side capacitance element, maysuffice to create the voltage source characteristics of the at least onefirst side converter circuit, irrespective of the actual number of firstside converter circuits.

According to a further embodiment, each of the at least one first sideconverter circuits comprises an H bridge circuit. An H bridge circuitcomprises four switches, in particular four transistors. Alternatively,it is possible that each of the first side converter circuits comprisesa capacitive half bridge circuit. Such a capacitive half bridge circuitcomprises two switches, for example two transistors, and one or twocapacitors.

According to a further embodiment, each of the plurality of second sideconverter circuits has a voltage source characteristic. The arrangementof one or more first side converter circuits with voltage sourcecharacteristics, one or more transformer circuits with current sourcecharacteristics and the second side converter circuits with voltagesource characteristics allows for providing power transfer symmetry inthe DC/DC conversion stage of the DC/AC converter. Also, the voltagesource characteristics of the second side converter circuits allow forgood coupling properties to the DC/AC conversion stage. In a particularembodiment, each of the plurality of second side converter circuitscomprises a second side capacitance element, with the second sidecapacitance elements being connected in series between the pair ofsecond side terminals.

According to a further embodiment, each of the plurality of second sideconverter circuits comprises an H bridge circuit. An H bridge circuitmay comprise four switches, in particular four transistors.

According to a further embodiment, the number of second side convertercircuits is one of 2, 4, 6 and 8. With these values, a good trade-offbetween dividing up the power transfer among several second sideconverter circuits, which leads to lower power transfer requirements forthe individual second side converter circuits as compared to previousapproaches, and preventing the wiring and control expenses from becomingexcessive can be achieved.

In a further embodiment, the at least one first side converter circuitis a plurality of first side converter circuits, with the plurality offirst side converter circuits being connected in parallel between thefirst side terminals. In this way, the power transfer is also divided upamong several first side converter circuits, such that the powertransfer requirements per first side converter circuit can also belowered on the first side of the DC/DC conversion stage as compared toprevious approaches. This dividing up of power transfer, both on thefirst and second sides, may also lead to faster response times of theDC/DC conversion stage, such that the DC/AC converter is better atadapting to varying operating conditions. According to a furtherembodiment, the number of first side converter circuits is equal to thenumber of second side converter circuits. In this way, a plurality ofpower transfer subsystems consisting of one first side convertercircuit, one transformer circuit and one second side converter circuitare formed. Such subsystems consisting of three circuits only areparticularly well-behaved and may be easily embedded into the effectivecontrolling of the DC/AC converter over a wide range of operatingconditions.

According to a further embodiment, the number of second side convertercircuits is equal to the number of first side converter circuitsmultiplied by N, with N being a natural number of at least 2. Inparticular, N second side converter circuits, N transformer circuits andone first side converter circuits may form a respective power transfersubsystem. In this way, manageable power transfer subsystems consistingof a limited number of circuits are formed, which are stillwell-behaved, but require less hardware than the 1:1 ratio between firstand second converter circuits as described above. The N transformercircuits of a respective power transfer subsystem may be connected inparallel. It is also possible to form such a power transfer subsystemfrom one first side converter circuit, one transformer circuit and Nsecond side converter circuits. The one transformer circuit may thencouple the one first side converter circuit to the N second sideconverter circuits via a transformer having one set of transformerwindings on the first side and N sets of transformer windings on thesecond side.

According to a further embodiment, each of the at least one first sideconverter circuits is controlled by a respective first side controlsignal and each of the plurality of the second side converter circuitsis controlled by a respective second side control signal. For thispurpose, the DC/AC converter may comprise a control circuit coupled tothe at least one first side converter circuit and the second sideconverter circuit and adapted to generate the first side control signalsand second side control signals. In particular, the first side controlsignals and the second side control signals are signals for controllingrespective H bridges.

According to a further embodiment, the first side control signals andsecond side control signals are controlled for being pulse signalshaving the same periods and duty cycles, with a phase relationshipbetween the first side control signals and the second side controlsignals being controlled for controlling a power transfer between thepair of first side terminals and the pair of second side terminals. Apulse signal may be understood as a signal alternating between twostates. In particular, the pulse signal may be a rectangular waveformsignal. With the control signals generally controlling switches of thefirst and second side converter circuits, the amplitude of the controlsignals is generally irrelevant, as long as proper switching of theswitches is ensured. In this way, only one kind of signal, in particularonly one kind of waveform, is generated by the control circuit forcontrolling the DC/DC conversion stage. The control of the DC/DCconversion stage is then exclusively carried out via the phaserelationship of different instances of that waveform. In this way, thecontrol system expenses are kept low, and the control system,influencing only the phase relationship, can act fast when reacting tooperating condition changes. According to a particular embodiment, thefirst side control signals and the second side control signals are pulsesignals having a duty cycle of substantially 50%.

According to a further embodiment, each second side converter circuit iscontrolled with respect to its associated first side converter circuit,with a phase shift between the second side control signal of therespective second side converter circuit and the first side controlsignal of its associated first side converter circuit controlling apower transfer. In this way, a separate control of each power transfersubsystem is achieved, such that different loading of the various powertransfer subsystems can be accounted for by the control of the DC/DCconversion stage. It is possible to control the phasing of each secondside converter circuit depending on the associated first side convertercircuit, such that a plurality of second side converter circuits thatare all associated with the same first side converter circuit can stillbe controlled independently from each other.

According to a further embodiment, a ratio of a second side voltage ofeach of the second side converter circuits and a voltage across the pairof first side terminals is controlled to be a transformer ratio of therespective one of the at least one transformer circuit coupling therespective second side converter circuit to the at least one first sideconverter circuit. In other words, the DC/AC converter, in particularits control circuit, is adapted to control each second side convertercircuit such that a voltage across its second side terminals is thevoltage across the first side terminals corrected by the transformerratio of the transformer circuit coupling the respective second sideconverter circuit to the at least one first side converter circuit. Theterm corrected may be substituted in the preceding sentence with theterm divided, if the transformer ratio of the respective transformer isdefined as the number of windings on the side of the first sideconverter circuit divided by the number of windings on the side of thesecond side converter circuit. By controlling the ratio between thevoltage across the first side terminals and across the second sideterminals of the second side converter circuits in this way, thereactive power losses of the DC/DC conversion stage can be kept to aminimum, such that the DC/DC conversion stage can be operated with veryhigh efficiency.

According to a further embodiment, each of the plurality of second sideconverter circuits is controlled separately. In this way, above ratiocan be maintained for each of the second side converter circuits, evenin the case of unequal loading of the second side converter circuitsfrom the DC/AC conversion stage and from the AC load connected thereto.

It is pointed out that the circuit structure of the DC/AC converter, asdescribed above and as claimed, allows for the advantages described whenbeing used with a wide variety of methods of controlling the DC/ACconverter, in particular with various methods of controlling the DC/ACconversion stage thereof. However, with the method of controlling theDC/AC conversion stage, as described herein, additional advantages and aparticularly good system behavior may be achieved.

Above mentioned problem is further solved by the method of controllingthe DC/AC conversion stage of the DC/AC converter in accordance with theclaims.

The claimed method of controlling the DC/AC conversion stage of theDC/AC converter comprises the steps of (a) providing a desired AC sidereference value; (b) setting a reference correction value; (c)calculating an AC side reference signal as a function of the desired ACside reference value and the reference correction value; (d) obtainingan actual AC side signal; and (e) calculating a converter control signalas a function of the AC side reference signal and the actual AC sidesignal; wherein the setting of the reference correction value is basedon a relation of the desired AC side reference value and the actual ACside signal.

The actual AC side signal is a signal that represents the electricbehavior at at least one of the AC terminals of the DC/AC converter. Itrepresents the actual electric behavior at the AC side of the DC/ACconverter. It can be a signal indicating the voltage behavior at the ACterminal or AC terminals under consideration. The AC side referencesignal represents a signal that serves as a reference to the actual ACside signal. It represents a behavior of the AC terminal(s) that thecontrol wants to achieve for the actual behavior at the AC terminal(s).The converter control signal is therefore calculated as a function ofthese two signals. The calculating of the converter control signal isreferred to as a lower control level of the DC/AC converter. With thecalculating of the converter control signal involving the actual AC sidebehavior of the DC/AC converter, a feedback control loop is established.The converter control signal is a signal controlling the actualconverter circuitry, in particular the half bridge converter(s) of theDC/AC converter.

The desired AC side reference value is a parameter which is acharacteristic value of the desired electric AC side behavior. With thedesired AC voltage being an alternating voltage and the desired currentpotentially having alternating components, the desired AC referencevalue may be an amplitude of the desired voltage or current waveformpresent at the AC terminals of the DC/AC converter.

As the reference correction value is set based on a relation of thedesired AC side reference value and the actual AC side signal, the ACside reference signal is a function of the desired AC side referencevalue and the actual AC side signal. In this way, a second feedbackcontrol loop is established, with the AC side reference signal being atthe output thereof and at the input of the lower control level, asdescribed above. Therefore, the setting of the reference correctionvalue is referred to as a higher control level of the DC/AC converter.It forms a controller for the controller.

With the reference correction value being based on the relation of thedesired AC side reference value and the actual AC side signal, there maybe instances during the operation of the control method where thereference correction value is set to not have an impact, such that theAC side reference signal corresponds to a signal representing thedesired behavior of the AC terminal(s) of the DC/AC converter. In otherinstances, however, the reference correction value may be set to such avalue that the AC side reference signal, while deviating from thedesired behavior of the AC terminal(s) of the DC/AC converter, is a moreeasily reachable target for the lower control level and/or makes iteasier for the lower level control to reach a behavior close to thedesired behavior at the AC terminal(s). In other words, the setting ofthe reference correction value allows for trading off control accuracyin terms of reaching exactly the desired AC side behavior for making itpossible or easier to reach an AC side behavior that is acceptably closeto the desired AC side behavior.

In this way, the higher level control helps the lower level control toreach its control target. The lower level control can make the actual ACside signal converge to the AC side reference signal also in cases whereit would be impossible without the provision of the reference correctionvalue. Therefore, the DC/AC converter is allowed to work reliably over awider range of operating conditions, as the control of the DC/ACconverter is allowed to converge in cases where it failed to converge inprevious approaches.

The control method works particularly effectively with the inventivecircuit structure of the DC/DC conversion stage, as described above,because it can work with the highly reliable reference point provided atthe connection between two of the second side converter circuits. Nosignal disturbances are introduced through the switching of thereference point half bridge converter present in previous approaches,such that the control handles a DC/AC conversion stage that is morepredictable and therefore more reliably controllable.

According to a further embodiment, the method is executed iteratively,with steps (d) and (e) being executed several times per every executionof the method. As with any control method, an iterative or continuousexecution of the calculations is carried out, such that the controltarget, namely the converging of the actual AC side signal to the ACside reference signal, can be achieved. Per every execution of steps (b)and (c), however, steps (d) and (e) are executed several times.Accordingly, the lower level control is given time to reach the controltarget, before the same is adjusted by the higher level control. In thisway, it is ensured that the controller for the controller onlyintervenes when the lower level control has difficulties reaching thecontrol target. If there are no such difficulties, no control accuracyis given up. For example, step (e) may be a continuous calculation ormay take place many times per period of the actual AC side signal,whereas step (b) may be carried out once or twice per period of theactual AC side signal.

According to a further embodiment, the setting of the referencecorrection value is done in such a way that a deviation of the actual ACside signal from the desired AC side reference value is decreased. Inother words, influence is taken on a control error present between theAC side reference signal and the actual AC side signal in such a way viathe higher level control of the AC side reference signal that theconverter control signal leads to a decrease in the deviation of theactual AC side signal from the desired behavior. In yet other words,steps (b) and (c) result in the AC side reference signal being adjustedin a way to minimize a deviation between a characteristic value of theactual AC side signal, such as an amplitude value of the actual AC sidesignal, and the desired AC side reference value.

In order to achieve above described decrease of the deviation of theactual AC side signal from the desired AC side reference value, thesteps (b) and (c) may result in an increase of a deviation between theAC side reference signal and the actual AC side signal. In other words,the AC side reference signal is controlled to have a greater deviationfrom the actual AC signal as compared a control method without thereference correction value. In this way, the higher level control sets atarget for the lower level control that is further removed than theactually desired behavior. As a consequence, the lower level control maytake more drastic measures to work towards its control target. In thisway, an operating point close to the desired AC side behavior may bereached faster. Therefore, the control target is worked towards in twoways. While step (e) controls the DC/AC converter circuitry in such away that the actual AC side signals approximates the AC side referencesignal, steps (b) and (c) aim at manipulating the AC side referencesignal in such a way that the actual AC side signal is brought close tothe AC side reference signal that would be present without the effect ofthe reference correction value in a more effective manner.

According to a further embodiment, the relation of the desired AC sidereference value and the actual AC side signal is a relation of thedesired AC side reference value and an amplitude of the actual AC sidesignal. In other words, the setting of the reference correction value isbased on a comparison of a characteristic parameter of the actual ACside signal, namely the amplitudes thereof, and the desired AC sidereference value. The desired AC side reference value may therefore be adesired AC side amplitude reference value. Accordingly, the actual ACside signal may not have to be analysed completely in order to berelated to the desired AC side reference value. In this way, thehardware and/or computing requirements for the higher control level arekept low.

According to a further embodiment, the relation of the desired AC sidereference value and the actual AC side signal is a difference betweenthe desired AC side reference value and an amplitude of the actual ACside signal, with the reference correction value being adjusted if anabsolute value of said difference between the desired AC side referencevalue and the amplitude of the actual AC side signal is above a presetcorrection threshold. The difference between the amplitude of the actualAC side signal and the desired AC side reference value may be comparedwith the preset correction threshold. In this way, it is ensured thatminor deviations of the actual AC side signal from the desired AC sidereference value do not lead to an intervention from the higher levelcontrol via the reference correction value. In particular, it isprevented that such an intervention would lead to another deviation ofthe actual AC side signal in the other direction, such that the higherlevel control would then adjust the reference correction value in theother direction as well. In such a way, a continuous toggling betweentwo reference correction values could arise, which is prevented via thepreset correction threshold.

According to a further embodiment, the reference correction value isadjusted by applying a preset increment value. In this way, a step sizefor adjusting the reference correction value is set. It allows thereforefor adjusting the AC side reference signal in preset step sizes.Accordingly, in an iterative process, it is tested how well thedeviation between the actual AC side signal and the desired AC sidereference value can be mitigated via the converter control signal, i.e.via the lower level control. Iteratively, it is checked if the deviationcan be eliminated with the new reference correction value. If not, thereference correction value is again adjusted by the preset incrementvalue. Applying a preset increment value is understood to be adding orsubtracting the preset increment value to the momentary referencecorrection value depending on a sign of the difference between thedesired AC side reference value and the amplitude of the actual AC sidesignal. The preset increment value may be the same or a different valuethan the preset correction threshold described above.

According to a further embodiment, the desired AC side reference valueis a desired voltage amplitude value and the reference correction valueis a voltage amplitude correction value, with the step of calculatingthe AC side reference signal comprising calculating a sum of the desiredvoltage amplitude value and the voltage amplitude correction value andmultiplying said sum with a desired waveform signal having a desired ACfrequency. The desired waveform signal may in particular be a sinusoidalwaveform signal.

While the present control method is particularly suitable for the powertransfer from the DC side to the AC side of the DC/AC converter, a powertransfer from the AC side to the DC side can also be effected. Asdiscussed above, the DC/AC converter may be a bi-directional DC/ACconverter.

According to a further embodiment, step (c) comprises generating adistortion smoothing signal, with the AC side reference signal beingcalculated as a function of the desired AC side reference value, thereference correction value and the distortion smoothing signal. Theswitching of the converters, in particular of the half bridgeconverters, of the DC/AC converter commonly leads to artifacts in the ACoutput of the DC/AC converter, such that the AC voltage/current at theAC terminal(s) does contain undesired signal components. These artifactscan be preemptively counter-measured by providing the distortionsmoothing signal. The artifacts are particularly severe for low dutycycles of the converter control signals, i.e. for short conduction timesin the converter switches, which might occur at low voltage values forthe actual AC side signal. Accordingly, the distortion smoothing signalmay be particularly designed to provide an improved behavior in thesescenarios, extending the well-behaved range of operating conditions ofthe DC/AC converter.

According to a further embodiment, the distortion smoothing signalcomprises at least one signal component having a frequency that is amultiple of a desired AC side frequency. In this way, the distortionsmoothing signal eliminates harmonics introduced by the switchingactions in the converter circuits. The distortion smoothing signal maybe comprised of one or of a plurality of signal components.

According to a further embodiment, the converter control signal controlsa switching pulse width for a converter half bridge. The convertercontrol signal may therefore be seen as a PWM signal (pulse widthmodulation signal).

According to a further embodiment, the DC/AC converter comprises aplurality of AC side terminals, with each of the plurality of AC sideterminals having a respective actual AC side signal, with each of theplurality of actual AC side signals being controlled separately.Accordingly, the control method as described above is carried outindividually per phase of the AC voltage system of the DC/AC converter.In other words, the actual AC side signal may be comprised of aplurality of phases, with each phase being controlled separately inaccordance with the method described herein. The AC voltage system ofthe DC/AC converter may have three phases, but it also may have anothernumber of phases.

According to a further embodiment, the converter control signal isobtained from a feed forward control signal, which is based on the ACside reference signal, and a feedback control signal, which iscalculated by applying a control algorithm to a difference between theAC side reference signal and the actual AC side signal. In this way, aclosed control loop is established that is accompanied by a feed forwardcontrol path, alleviating some of the control burden of the feedbackcontrol path. The control algorithm of the feedback control path maycomprise a proportional gain element and a generalized integrationelement.

According to a further embodiment, the feed forward control signal maybe calculated as a function of the AC side reference signal and anactual DC side signal. In particular, the feed forward control signalmay be calculated as a function of the AC side reference signal and anactual DC side voltage value. The AC side reference signal may be scaledby the actual DC side voltage value. It may be divided by the actual DCside voltage value and multiplied by a feed forward multiplicationfactor. In this way, it is ensured that the converter control signal isadjusted to the DC voltage present at the converter half bridges, suchthat the closing times are adjusted to the current operating conditions.The actual DC side voltage value refers to the voltage value present atthe DC side of the converter circuits of the DC/AC converter. If theDC/AC converter comprises a DC/DC conversion stage and a DC/ACconversion stage, the actual DC side voltage value is the DC sidevoltage of the DC/AC conversion stage. It is the DC voltage at that sideof the DC/DC conversion stage that is coupled to the DC/AC conversionstage.

According to a further embodiment, the DC/DC conversion stage has adesired operating point and the converter control signal is dependent ona deviation of the DC/DC conversion stage from the desired operatingpoint. In this way, the DC/AC conversion stage may be controlled in sucha way that its control accuracy is neglected in situations where theDC/DC conversion stage cannot support its desired operating point infavor of the DC/DC conversion stage recuperating its desired operatingpoint. In this way, the DC/AC converter as a whole may be able to reactfaster to operating condition changes and the control problems arisingtherefrom. In particular, the minimum necessary voltage levels at theDC/DC conversion stage may be maintained, such that the DC/AC converteras a whole is not in risk of experiencing an emergency power-downcondition. Therefore, this feature again helps keeping the DC/ACconverter working over a wide range of operating conditions, only givingup temporary control accuracy.

According to a further embodiment, the DC/DC conversion stage has adesired operating point, the method further comprising the steps ofcalculating a reference signal adjustment value as a function of adeviation of the DC/DC conversion stage from the desired operatingpoint; and, after step (c) and before step (e), adjusting the AC sidereference signal on the basis of the reference signal adjustment value.This is a way how the momentary operating conditions of the DC/DCconversion stage can be accounted for. The control accuracy of the DC/ACconversion stage, namely the amplitude accuracy of the actual ACbehavior, can temporarily be traded off against a recuperation of thedesired operating point by the DC/DC conversion stage.

According to a further embodiment, the reference signal adjustment valueis set to 1 when the DC/DC conversion stage does not deviate more than apreset deviation threshold from the desired operating point.Accordingly, this control mechanism only kicks in when the DC/DCconversion stage deviates a significant amount from its desiredoperating point.

According to a further embodiment, the DC/DC conversion stage has adesired operating point, the method further comprising the steps ofproviding a desired AC frequency value; calculating an AC frequencyadjustment value as a function of a deviation of the DC/DC conversionstage from the desired operating point; and controlling an AC frequencyof the DC/AC converter on the basis of the desired AC frequency valueand the AC frequency adjustment value. This is a further way how themomentary operating conditions of the DC/DC conversion stage can beaccounted for. Again, the control accuracy of the DC/AC conversionstage, namely the frequency accuracy of the actual AC behavior, cantemporarily be traded off against a recuperation of the desiredoperating point by the DC/DC conversion stage. The adjustment of thefrequency can also help to lower the power supplied from the DC/ACconverter to the AC network connected thereto, and can thus help theDC/DC conversion stage to be less loaded temporarily and recuperate.

According to a further embodiment, the step of controlling the ACfrequency of the DC/AC converter comprises the step of multiplying thedesired AC frequency value with the AC frequency adjustment value, withthe AC frequency adjustment value being 1 when the DC/DC conversionstage does not deviate more than a preset deviation threshold from thedesired operating point.

With the provision of the reference signal adjustment value and the ACfrequency adjustment value, it can be influenced which one of thefrequency of the AC voltage system or the amplitude of the AC voltagesystem is to be adjusted when the DC/DC conversion stage does notoperate as desired. In AC voltage systems where the DC/AC converter isthe only power source for the AC voltage system, also referred to aspassive networks, preference may be given to a change in amplitude. InAC voltage systems where the DC/AC converter is one of a plurality ofpower sources for the AC voltage system, also referred to as activenetworks, preference may be given to a change in frequency.

According to a further embodiment, the method comprises the step ofcalculating a DC/DC conversion stage performance metric indicative ofthe deviation of the DC/DC conversion stage from the desired operatingpoint, which comprises the steps of dividing a voltage at a second sideof the DC/DC conversion stage by a voltage at a first side of the DC/DCconversion stage and multiplying the result with a desired DC/DCconversion ratio. In this way, the DC/DC conversion stage performancemetric represents a ratio of the actual DC voltage at the DC side of theDC/AC conversion stage and the desired DC voltage at the DC side of theDC/AC conversion stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with regard to theexemplary embodiments shown in the accompanying figures, in which:

FIG. 1 shows a block diagram of a DC/AC converter.

FIG. 2 shows a previous approach implementation of a DC/AC conversionstage of a DC/AC converter.

FIG. 3 shows a previous approach implementation of a DC/DC conversionstage of DC/AC converter.

FIG. 4 shows a circuit diagram of a DC/AC converter according to anexemplary embodiment of the invention.

FIG. 5 shows a circuit diagram of a DC/AC converter according to anotherexemplary embodiment of the invention.

FIG. 6 shows a timing diagram of the control signals applied to theDC/DC conversion stage according to an exemplary embodiment of theinvention.

FIG. 7 shows a block diagram illustrating the control of the DC/DCconversion stage according to an exemplary embodiment of the invention.

FIG. 8 shows a block diagram illustrating the control of the DC/ACconversion stage according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows a circuit diagram of a DC/AC converter 2 according to anexemplary embodiment of the invention. The DC/AC converter 2 comprises aDC/DC conversion stage 4 and a DC/AC conversion stage 6. Elementsidentical or similar to the corresponding elements of FIGS. 1 to 3 aredenoted with the same reference numerals for the sake of an easierunderstanding.

The DC/DC conversion stage 4 comprises a pair of first side terminals 10and a pair of second side terminals 12. As the pair of first sideterminals 10 are the DC end of the DC/AC converter 2, they are alsoreferred to as DC side terminals of the DC/DC conversion stage 4.Moreover, as the second side terminals 12 form the connection point tothe DC/AC conversion stage 6, they are also referred to as AC sideterminals of the DC/DC conversion stage 4.

Two first side converter circuits 20, 22 and a first side capacitor 50are respectively coupled between the pair of first side terminals 10. Inother words, the first side converter circuits 20, 22 and the first sidecapacitor 50 are coupled in parallel between the first side terminals10. Each of the first side converter circuits 20, 22 comprises fourtransistors coupled in an H bridge arrangement. The transistors of thefirst side converter circuit 20 are denoted T_(20.1) to T_(20.4). Thetransistors of the first side converter circuit 22 are denoted T_(22.1)to T_(22.4). In the circuit diagram of FIG. 4, the transistors arerepresented by a switch and an antiparallel diode, respectively,indicating that these transistors are MOSFETs in the exemplaryembodiment of FIG. 4. The transistors may also be insulated gate bipolartransistors coupled with antiparallel diodes or other suitable powerelectronic switches having suitable properties.

Due to the provision of the first side capacitor 50, the first sideconverter circuits 20, 22 have voltage source characteristics. It couldalso be stated that the group of the first side converter circuits 20,22 has a voltage source characteristic. It is pointed out that it wouldbe electrically equivalent to provide a capacitor in each of the firstside converter circuits 20, 22 making it clear that both of the firstside converter circuits 20, 22 have voltage source characteristics.

Two second side converter circuits 30, 32 are coupled in series betweenthe pair of second side terminals 12. Each of the second side convertercircuits 30, 32 comprises four transistors coupled in an H bridgearrangement as well as a second side capacitor. The transistors of thesecond side converter circuit 30 are denoted T_(30.1) to T_(30.4), whilethe second side capacitor of the second side converter circuit 30 isdenoted with reference numeral 60. The transistors of the second sideconverter circuit 32 are denoted with reference numerals T_(32.1) toT_(32.4), while the second side capacitor of the second side convertercircuit 32 is denoted with reference numeral 62. The transistors of thesecond side converter circuits are depicted in the same way as thetransistors of the first side converter circuits, indicating that theyare MOSFETs in the exemplary embodiment of FIG. 4. The transistors mayalso be insulated gate bipolar transistors coupled with antiparalleldiodes or other suitable power electronic switches having suitableproperties.

The second side converter circuits 30, 32 are coupled at a connectionpoint 34, such that they are arranged in series between the pair ofsecond side terminals 12. Due to the provision of the second sidecapacitors 60, 62 in parallel with the respective H bridge circuits, thesecond side converter circuits 30, 32 have voltage sourcecharacteristics. Also, the group of second side converter circuits 30,32 has a voltage source characteristic.

The DC/DC conversion stage 4 further comprises two transformer circuits40, 44. The transformer circuit 40 comprises a transformer 41 and aninductance element 42 coupled in series with the transformer 41. Thetransformer circuit 44 comprises a transformer 45 and an inductanceelement 46 coupled in series with the transformer 45. The provision ofthe inductance elements 42, 46 ensures that the transformer circuits 40,44 have current source characteristics. In the exemplary embodiment ofFIG. 4, the inductance elements 42, 46 are separate, discrete circuitelements. However, it is also possible that these inductance elementsare parasitic inductances of other circuit elements, for example of thetransformers 41 and 45. However, the inductance elements, parasitic ornot, are chosen to have enough inductance to provide for the currentsource characteristic of transformer circuits 40, 44.

The transformer circuit 40 couples the H bridge of the first sideconverter circuit 20 to the H bridge of the second side convertercircuit 30. The transformer circuit 44 couples the H bridge of the firstside converter circuit 22 to the H bridge of the second side convertercircuit 32.

The DC/AC conversion stage 6 of the DC/AC converter 2 is coupled betweenthe second side terminals 12 of the DC/DC conversion stage 4 and threeAC terminals 80, 82 and 84. The DC/AC conversion stage 6 comprises thethree AC terminals, because the exemplary AC voltage system coupled tothe exemplary DC/AC converter 2 is a three phase AC voltage system. Ofcourse, depending on the number of phases required for whatever systemis coupled to the AC terminals of the DC/AC converter 2, a differentnumber of AC terminals may be present. The DC/AC conversion stage 6further comprises a neutral terminal 86, which serves as a referenceterminal to the AC terminals 80, 82 and 84, in particular as a centerpoint of a three phase star AC configuration.

The DC/AC conversion stage 6 of the exemplary DC/AC converter 2 of FIG.4 further comprises three half bridge converters 70, 72 and 74. Theseare associated with the AC terminals 80, 82 and 84, respectively, andselectively allow for current flow between the second side terminals 12and the AC terminals. The half bridge converters 70, 72 and 74 arerespectively coupled between the second side terminals 12. Each halfbridge converter comprises a transistor pair coupled in series, with thecenter point between the two transistors being coupled to the respectiveAC terminal Each of the transistors is again depicted as a switch and adiode connected in parallel, indicating that they are MOSFETs in theembodiment of FIG. 4. The transistors may also be insulated gate bipolartransistors coupled with antiparallel diodes or other suitable powerelectronic switches having suitable properties.

In the particular embodiment of FIG. 4, the half bridge converters 70,72 and 74 are coupled to the AC terminals 80, 82 and 84 through a filter78. The filter 78 comprises inductive elements as well as capacitiveelements conditioning the AC voltages at terminals 80, 82, 84 inaccordance with the requirements of a particular application. The filter78 provides for an LC filtering element between each of the AC terminalsand the neutral terminal 86 as a reference. However, it is pointed outthat the filter 78 is an optional feature and may be dispensed with.

The neutral terminal 86 forms a neutral phase point of the DC/ACconversion stage 6, forming a reference point for the AC terminals 80,82 and 84. It is 35 coupled to and interacts with the AC terminals 80,82 and 84 via the filter 78.

The neutral terminal 86 is further coupled to the connection point 34 ofthe DC/DC conversion stage 4. In the embodiment of FIG. 4, the neutralterminal is directly connected to the connection point 34, without anyother circuit elements arranged therebetween.

The operation of the DC/AC converter 2 is described as follows.

As the DC/DC conversion stage 4 is electrically symmetric between thepair of first side terminals 10 and the pair of second side terminals12, identical operating behaviour can be achieved and identical controlalgorithms can be used for a power transfer from the first sideterminals to the second side terminals and from the second sideterminals to the first side terminals. The symmetry is achieved throughthe arrangement of converter circuits with voltage sourcecharacteristics at the first side terminals as well as the second sideterminals, with transformer circuits with current source characteristicbeing arranged therebetween.

The DC/DC conversion stage 4 is controlled in such a way that thevoltage across each of the second side capacitors 60, 62 equals thevoltage across the first side capacitor 50 divided by the transformerratio of the respective transformer 41, 45. In other words, the voltageacross the second side capacitance element 60 is controlled to be equalto the voltage across the first side capacitor 50, which is V_(DC,Prim),divided by the transformer ratio N_(T,41) of transformer 41, which isdefined as the ratio of first side transformer windings N_(Prim,41) andsecond side transformer windings N_(Sec,41) of the transformer 41, i.e.as N_(Prim,41)/N_(Sec,41). Analogously, the voltage across the secondside capacitor 62 is controlled to be equal to the voltage across thefirst side capacitor 50 divided by the transformer ratio N_(T,45) oftransformer 45, which is defined as the ratio of first side transformerwindings N_(Prim,45) and second side transformer windings N_(Sec,45) ofthe transformer 45, i.e. as the ratio N_(Prim,45)/N_(Sec,45).Consequently, the voltage across the second side terminals, namelyV_(DC,Sec), is controlled to be V_(DC,Prim)*(1/N_(T,41)+1/N_(T,45)).

In the exemplary embodiment of FIG. 4, both N_(T,41) and N_(T,45) are2/1. Therefore, above formula yields that the voltage across the secondside terminals 12, V_(DC,Sec), is controlled to equal the voltage acrossthe first side terminals 10, V_(DC,Prim). Accordingly, the depictedembodiment allows for a galvanic isolation between the first sideterminals 10 and the second side terminals 12 without a change involtage therebetween.

Depending on the particular application for the DC/AC converter, avoltage ratio other than 1 between the first side terminals 10 and thesecond side terminals 12 may be desirable. The ratios of the respectivetransformer windings as well as an arrangement of further second sideconverter circuits provide for a large number of degrees of freedom thata designer may use to adapt the DC/AC converter to the particular usagescenario. In a case where all transformer circuits have transformerswith the same transformer ratios, denoted N_(T), the voltage across thesecond side terminals 12 equals the voltage across the first sideterminals 10 divided by the transformer ratio N_(T) and multiplied bythe number of second side converter circuits of the particularimplementation.

The control of the DC/DC conversion stage 4 is described with respect toa subsystem thereof comprising the first side converter circuit 20, thetransformer circuit 40, and the second side converter circuit 30. Forexemplary purposes, a power transfer from the first side convertercircuit 20 through the transformer circuit 40 to the second sideconverter circuit 30 is described. As explained above, due to thesymmetry of the DC/DC conversion stage 4, a power transfer from thesecond side converter circuit 30 through the transformer circuit 40 tothe first side converter circuit 20 can be effected in an analogousmanner. Reference is also made to FIG. 6, wherein a timing diagram ofthe control signals to the transistors of the first side convertercircuit 20 and the second side converter circuit 30 is depicted. A“high” state of these control signals indicates that the switch of therespective transistor is closed, whereas a “low” state indicates thatthe switch of the respective transistor is open.

As can be seen in FIG. 6, the transistors of the first side convertercircuit 20 are controlled in such a way that the diagonals of the Hbridge are put into a conductive state in an alternating manner Both ofthe diagonals are controlled to be conductive during 50% of the time. Inother words, two complementary PWM signals are supplied to the twodiagonals of the H bridge, with the PWM signals having a duty cycle of50%. The control alternates between putting the transistors T_(20.1) andT_(20.4) on the one hand and the transistors T_(20.2) and T_(20.3) onthe other hand into a conductive state.

The diagonals of the second side converter circuits 30 are alsocontrolled by two complementary PWM signals having a duty cycle of 50%,as shown in FIG. 6. In the setup of FIG. 4, the phase shift between thecomplementary PWM signals controlling the first side converter circuit20 and the complementary PWM signals controlling the second sideconverter circuit 30, denoted as φ₁ controls the direction and amount ofpower transfer between the first side converter circuit 20 and thesecond side converter circuit 30.

For example, when the DC/AC converter 2 is operated to supply AC powerto an AC load coupled to the AC terminals 80, 82 and 84, a powertransfer from the first side converter circuit 20 through thetransformer circuit 40 to the second side converter circuit 30 takesplace. In this case, if the second side capacitor 60 of the second sideconverter circuit 30 deviates from its desired value, namely the voltageacross the first side capacitor 50 divided by the transformer ratio ofthe transformer 40, the phase shift φ₁, is adjusted, such that theadjusted power transfer ensures a return of the voltage across thesecond side capacitor 60 to the desired value. In other words, a controlalgorithm is established, wherein the deviation of the voltage acrossthe second side capacitor 60 from the desired value represents thecontrol error and the phase shift φ₁, represents the correctingvariable.

FIG. 7 shows a block diagram of the control of both subsystems of theDC/DC conversion stage 4 of FIG. 4. The correcting variable φ₁, is usedfor controlling the first subsystem consisting of the first sideconverter circuit 20, the transformer circuit 40 and the second sideconverter circuit 30. The correcting variable φ₂ is used for controllingthe second subsystem consisting of the first side converter circuit 22,the transformer circuit 44 and the second side converter circuit 32. Asthe transformer ratios of the transformer 41 and the transformer 45 arethe same in the exemplary embodiment of FIG. 4, namely N_(T), thedesired voltages across the second side capacitors 60 and 62 are thesame, namely V_(DC,Prim)/N_(T). The actual voltages across the secondside capacitors 60, 62, denoted V_(DC,Sec,60) and V_(DC,Sec,62). arerespectively subtracted from the desired voltages across thesecapacitors, such that a respective control error is determined. Therespective control errors are fed into respective control blocks 90, 92,wherein the respective phase shifts for the first and second subsystemsare calculated on the basis of current and previous control errors. Inthe exemplary embodiment of FIG. 7, PI control algorithms (proportionalintegral control algorithms) are implemented in the control blocks 90and 92. As is apparent to a person skilled in the art, however, othercontrol algorithms may be suitable depending on the particularapplication and desired behaviour of the DC/AC converter.

FIG. 7 shows the additional block 94, which generates an offset valueφ_(Offset). The actual control signals for the first and secondsubsystems, namely φ₁ and φ₂, are determined on the basis of the outputsof control blocks 90, 92 and the value of φ_(Offset). In this way, anadditional degree of freedom for conditioning the power transfer betweenthe first side terminals 10 and the second terminals 12 is given. It ispointed out that block 94 is an optional feature of the control depictedin FIG. 7.

In the exemplary embodiment of FIG. 4, the voltages across the twosecond side capacitors 60 and 62 are controlled to be the same.Accordingly, the connection point 34 between the two second sideconverter circuits 30 and 32 has, in operation, a potential in themiddle of the two potentials present at the pair of second sideterminals 12. In other words, the potential of the connection point 34is the average of the potentials at the second side terminals 12. In yetother words, the connection point 34 is controlled to be a point ofneutral potential with regard to the pair of second side terminals 12.As such a point of neutral potential, the connection point 34 forms areference point for the AC voltage system. The neutral terminal 86 isconnected directly to the connection point 34, such that the referencepoint of the AC voltage system is obtained without the provision of afurther half bridge converter between the pair of second side terminals12. With the connection point 34 coupled to the neutral terminal 86, theAC voltages at AC terminals 80, 82 and 84 can alternate freely betweenthe two voltage levels present at the pair of second side terminals 12,while the connection between the neutral terminal 86 and the connectionpoint 34 ensures that the reference voltage of the AC voltage systemequals the midpoint between the two voltage levels present at the pairof second side terminals 12. In an alternative embodiment, a periodicasymmetry of voltages at the pair of second side terminals 12 withregard to the voltage at the connection point 34 may be effected by thecontrol. In this way, a higher AC voltage level may be achieved at theAC terminals 80, 82 and 84. It is also possible to achieve a given ACvoltage level with a lower duty cycle at the half bridge converters 70,72 and 74 in this way.

Reference is now made to FIG. 5, which shows a DC/AC converter 2 havinga slightly different circuit structure than the DC/AC converter 2 ofFIG. 4. In particular, the DC/DC conversion stage 4 of the DC/ACconverter 2 of FIG. 5 only comprises one first side converter circuit 24instead of the two first side converter circuits 20, 22 of FIG. 4. Thefirst side converter circuit 24 comprises an H bridge circuit, which inturn comprises the four MOSFETs T_(24.1) to T_(24.4), depicted as aparallel connection of a switch and a diode, respectively. The twotransformer circuits 40, 44 are coupled to the H bridge of the firstside converter circuit 24 in parallel, i.e. their terminals are coupledto the same connection points within the first side converter circuit24. Accordingly, all power transferred between the pair of first sideterminals 10 and the pair of second side terminals 12 flows through thefirst side converter circuit 24, with that power being split up betweenthe transformer circuit 40 and the second side converter circuit 30 onthe one hand and the transformer circuit 44 and the second sideconverter circuit 32 on the other hand. Accordingly, the DC/DCconversion stage of the DC/AC converter 2 can be realized with fewercomponents at the expense of the first side converter circuit 24 beingrequired to allow greater current levels in order to reach the samepower transfer capability. Still, the H bridge circuit of the two secondside converter circuits 30, 32 can be controlled independently withrespect to the first side converter circuit 24, such that individualpower transfer amounts can be established in order to account fordifferent power requirements at the two second side capacitors 60 and62, for example caused by non-equally distributed load conditions at thethree AC phases 80, 82 and 84.

The DC/AC converter 2 of FIG. 5 may further be modified in that only onetransformer circuit is present. This one transformer circuit has atransformer comprising one set of windings coupled to the first sideconverter circuit 24 and two sets of windings, each of which is coupledto one of the two second side converter circuits 30, 32.

The control of the DC/AC conversion stage 6 of the DC/AC converter 2 (ofboth FIG. 4 and FIG. 5) is now described with reference to FIG. 8. Inparticular, FIG. 8 describes the control of the DC/AC conversion stage 6for a load being coupled to the AC side of the DC/AC converter 2, i.e.for the power transfer direction being from the DC side to the AC side.FIG. 8 is a block diagram illustrating how the control signal for one ofthe half bridge converters 70, 72 and 74 of the DC/AC conversion stage 6is obtained. In particular, FIG. 8 shows how the PWM signal M_(AC,1) isobtained, which controls the half bridge converter 70 and through thecontrol of which the AC voltage between the terminals 80 and 86 iscontrolled. As this AC voltage phase is also referred to as the first ACvoltage, all variables given in FIG. 8 that may be different among thethree AC voltage phases are denoted with the index 1.

The method starts with providing a desired voltage amplitude value ofthe AC voltage between terminals 80 and 86, denoted V_(Amp). In moregeneral terms, this value is referred to as the desired AC sidereference value. In the block diagram of FIG. 8, function block 200 setsand outputs V_(Amp).

Further, a reference correction value, denoted V_(Amp,Corr,1) iscalculated. The calculation thereof is carried out by function block202, with the details thereof being explained below. The referencecorrection value V_(Amp,Corr,1) is added to the desired voltage valueV_(Amp).

The sum of the desired voltage value V_(Amp) and the referencecorrection value V_(Amp,Corr,1) is then multiplied with the waveformdesired to be present at the AC terminal 80. In the exemplary embodimentof FIG. 8, this wave form is a sinusoidal wave having the angularfrequency ω and the phase shift α₁. It is provided by function block204. The result of this multiplication can therefore be written as(V_(Amp)+V_(Amp,Corr,1))*sin(ωt+α₁).

The exemplary embodiment of FIG. 8 further comprises a distortionsmoothing signal creator 206, generating a distortion smoothing signalV_(Dist,1). The provision of the distortion smoothing signal V_(Dist,1)is based on the idea that the switching actions in the half bridgeconverters 70, 72 and 74 give rise to distortions in the resulting ACvoltages present at AC terminals 80, 82 and 84. By providing adistortion smoothing signal for the control of these half bridgecircuits, effective countermeasures against the switching distortionsare provided within the DC/AC converter control.

The distortion smoothing signal V_(Dist,1) in turn comprises a pluralityof signal components, with these signal components being harmonics ofthe sinusoidal signal representing the desired waveform of the ACsignal, which is provided by function block 204. In connection with theexemplary embodiment of FIG. 8, the general case of N harmonic signalcomponents is described. However, for a better intelligibility of FIG.8, only the function blocks responsible for the generation of two signalcomponents are actually depicted. Every distortion smoothing signalcomponent is derived from the multiplication of its amplitude and itswaveform. The function block 206 _(1,Amp) provides the amplitudeV_(1,Amp,1) of a distortion smoothing signal component having the samefrequency as the frequency of the desired AC frequency, namely ω, whilethe function block 206 _(1,WF) provides the sinusoidal waves form havingthe frequency ω. The function block 206 _(N,Amp) provides the amplitudeV_(N,Amp,1) of an N-th harmonic distortion smoothing signal componenthaving N times the frequency of the desired AC waveform, namely Nω,while the function block 206 _(N,WF) provides the sinusoidal waveformhaving the frequency Nω. The dots depicted between the function blocks206 _(1,WF) and 206 _(N,Amp) indicate that other harmonic signalcomponents may also be present in the exemplary embodiment of FIG. 8.All of the harmonic signal components have the same phase angle α₁, asthe desired AC voltage waveform, such that they represent in-phaseharmonics thereof.

The harmonic distortion smoothing signal components are added to yieldthe distortion smoothing signal V_(Dist,1). The distortion smoothingsignal V_(Dist,1) is then added to the term(V_(Amp)+V_(Amp,Corr,1))*sin(ω+α₁) in order to yield a reference signalfor the AC voltage present at AC terminal 80, denoted V_(AC,Set,1),which is also referred to as AC side reference signal in more generalterms.

In other embodiments, only a particular subset of harmonics may bepresent in the distortion smoothing signal V_(Dist,1), depending on theartifacts introduced by the half bridge converters. In particular, in anexemplary embodiment of the invention, there may be only the third orfifth or seventh harmonic signal component present in the distortionsmoothing signal V_(Dist,1). It is also possible that the distortionsmoothing signal creator 206 is entirely dispensed with.

The AC side reference signal V_(AC,Set,1) is then adjusted via amultiplication with a reference signal adjustment value P_(Adj,Amp)before being fed to a feed-forward control path 208 and a feedbackcontrol path 212. The result of the multiplication is denotedV_(AC,Set,1). The nature and calculation of the reference signaladjustment value P_(Adj,Amp) will be explained later. It is pointed out,however, that the multiplication with this reference signal adjustmentfactor is an optional feature of the exemplary embodiment of FIG. 8.When eliminating this multiplication, V′_(AC,Set,1) equals V_(AC,Set,1).

In the feed-forward control path 208, the AC side reference signalV′_(AC,Set,1) is scaled by the voltage across the second side terminals12, namely V_(DC,Sec). In particular, the AC side reference signalV′_(AC,Set,1) may be multiplied with the factor A/V_(DC,Sec), wherein Ais a multiplication constant. This multiplication is carried out byfunction block 210. In this way, it is ensured that the function block218, which will be described below, takes into account the momentaryvoltage conditions at the second side terminals 12 and controls the halfbridge converters accordingly. Alternatively, an appropriate scaling maybe performed in the function block 218 itself, such that the functionblock 210 in the feed forward control path 208 can be dispensed with.

In the feedback control path 212, a difference between the AC sidereference signal V′_(AC,Set,1) and an actual AC voltage at the ACterminal 80, denoted V_(AC,Act,1) also referred to as an actual AC sidesignal, is calculated. This difference represents a control error of themethod of controlling the DC/AC converter 2. The control error is fed totwo control elements, namely a proportional gain element 214 and ageneralized integration element 216. A generalized integration elementis an integration control element that is adapted to properly take intoaccount control errors of sinusoidal signals, preventing a cancellingout of control errors for subsequent half waves of the sinusoidalsignal. The output signals of the two control elements 214 and 216 arethen added and jointly form a feedback control signal.

The feed-forward control signal and the feedback control signal are thencombined by addition. The combined control signal represents amodulation level and is translated into a control signal for the halfbridge converter 70 by a function block 218. The control signal for thehalf bridge converter is denoted M_(AC,1). It results in a particularsequence of open/closed states of the switches in the half bridgeconverter 70 and may therefore be seen as a PWM signal.

Now, reference is made again to the function block 202 and thegeneration of the reference correction value V_(Amp,Corr,1). The goalpursued by the provision of the reference correction valueV_(Amp,Corr,1) is to help the control method for the DC/AC converter 2in such a way that the desired AC side behaviour may be slightlymodified in order for the control method to reach an AC side behaviorclose to the unmodified desired AC side behavior more easily, therebyimproving the response time of the control method, while accepting aslight deviation from the desired AC side behaviour within tolerablelimits. The provision of the reference correction value V_(Amp,Corr,1)can therefore be seen as a higher level controller for the lower levelcontroller consisting of feed-forward control path 208 and feedbackcontrol path 212, such that a two level control is formed.

For the purpose of providing the reference correction valueV_(Amp,Corr,1), the actual AC side signal V_(AC,Act,1) is compared tothe desired voltage value V_(Amp) in intervals. In the exemplaryembodiment of FIG. 8, this comparison takes place twice per signalperiod of the actual AC side signal V_(AC,Act,1), namely at the positiveand negative maximum points thereof. In other words, the amplitude ofthe actual AC side signal V_(AC,Act,1) is compared to the desiredvoltage value V_(Amp). From this comparison, the value V_(Amp,Corr,1) isobtained in such a way that the AC side reference signal V′_(AC,Set,1)is adjusted to reduce a deviation between the amplitude of the actual ACside signal V_(AC,Act,1) and the desired amplitude voltage value V_(Amp)at the AC terminal 80. In other words, V_(Amp,Corr,1) is calculated insuch a way that the control error fed to the feedback control path 212is adjusted. In the exemplary embodiment of FIG. 8, the referencecorrection value V_(Amp,Corr,1) is set in such a way that the controlerror fed to the feedback control path 212 is increased. The lower levelcontroller in the feedback control path 212 therefore faces a moreremoved control target and takes more drastic control measures. In thisway, the controlling of the DC/AC conversion stage 6 is adapted in sucha way that the deviation between the actual AC side signal V_(AC,Act,1)and the desired amplitude voltage value V_(Amp) can be decreased moreeasily and faster than without the provision of the reference correctionvalue V_(Amp,Corr,1). Therefore, an efficient control can be achievedover a wider range of operating conditions. In particular, dynamicoperating conditions can be dealt with in an improved way.

The calculation of the reference correction value V_(Amp,Corr,1) isexplained in more detail. The control method starts with the referencecorrection value being set to zero. During operation, it is determinedif the amplitude of the actual AC side signal V_(AC,Act,1) differs fromthe desired amplitude voltage value V_(Amp) more than a presetcorrection threshold V_(AC,Delta). In mathematical terms, it isdetermined if |Amplitude (V_(AC,Act,1))−V_(Amp)|>V_(AC,Delta). If thisis the case, V_(Amp,Corr,1) is incremented or decremented, depending onwhich one of the amplitudes of the actual AC side signal V_(AC,Act,1)and the desired amplitude voltage value V_(Amp) is greater. Inparticular, the reference correction value V_(Amp,Corr,1) is incrementedor decremented by a preset increment value, which is also V_(AC,Delta)in the exemplary embodiment of FIG. 8. However, it is possible that thepreset increment value is different from the preset correction thresholdV_(AC,Delta). By providing the reference correction value V_(Amp,Corr,1)in this way, the overall control accuracy is traded off against a properfunctioning over a wider range of operational scenarios. In particular,the described higher level control for the lower level control allowsthe DC/AC converter 2 to find a stable operating point close to thedesired AC side behavior that potentially could not have been foundwithout allowing a certain deviation from the desired AC side referencevalue.

It is pointed out that the control error to be fed to the feedbackcontrol path 212 is continuously calculated over the whole period of theAC side reference signal V′_(AC,Set,1) and the actual AC side signalV_(VAC,Act,1), whereas the reference correction value V_(Amp,Corr,1) isadjusted at most twice per period in the exemplary embodiment of FIG. 8.Accordingly, the controller comprising the feedback control path 212 hasthe chance to correct the control error before the control for thecontrol, implemented via the reference correction value V_(Amp,Corr,1,)starts overruling. Therefore, it is ensured that an attempt at reachingthe desired AC side behaviour is made before the control for the controlallows a loosening of the control goal. This may also be reached bycalculating the reference correction value V_(Amp,Corr,1) only after apreset delay time after a start-up or a change of the operatingconditions of the DC/AC converter 2. Also, the reference correctionvalue V_(Amp,Corr,1) may then be adjusted again only after the presetdelay time

Reference is now made to a high level means of influencing the controlof the DC/AC conversion stage 6 based on the momentary behavior of theDC/DC conversion stage 4. At the function block 220, a performancemetric indicative of a deviation of the DC/DC conversion stage 4 from adesired operating point is calculated. In the exemplary embodiment ofFIG. 8, the DC/DC conversion stage performance metric is defined by aratio of the actual voltage across the pair of second side terminals 12of the DC/DC conversion stage 4, namely V_(DC,Sec), and a desiredvoltage across the pair of second side terminals 12 of the DC/DCconversion stage 4. The desired voltage is V_(DC,Prim)/N, with N beingthe transformer ratio N_(T) of the transformers of the transformercircuits of the DC/DC conversion stage 4 divided by the number of secondside converter circuits N_(Sub), assuming that all transformers have thesame transformer ratio N_(T). The stated expression of the desiredvoltage across the pair of second side terminals 12 is based on abovedescribed control scheme of controlling the voltage across the secondside capacitors to be the voltage across the first side capacitordivided by the transformer ratio. Accordingly, the formula for theperformance metric is (V_(DC,Sec)*N_(T))/(V_(DC,Prim)*N_(Sub)) in theexemplary embodiment of FIG. 8. However, it is pointed out that otherperformance metrics may also be used. For example, a difference betweenthe actual voltage across the pair of second side terminals 12 and thedesired voltage across the pair of second side terminals 12 may beinvolved in generating such a performance metric.

When the DC/DC conversion stage 4 operates at the desired operatingpoint, the quotient of (V_(DC,Sec)*N_(T))/(V_(DC,Prim)*N_(Sub))equals 1. When the DC/DC conversion stage 4 deviates from the desiredoperating point, the performance metric deviates from the value of 1. Asgraphically indicated in connection with function block 222, it is thendetermined if the calculated performance metric deviates more thanV_(DC,Sec,Delta) or −V_(DC,Sec,Delta) from the value of 1. In moregeneral terms, it is determined if the performance metric deviates inabsolute terms more than a preset deviation threshold from its value atthe desired operating point. The function block 222 then calculates anoutput based on the deviation. In case the deviation is below the presetdeviation threshold, the function block 222 outputs the value 1. In casethe absolute amount of the deviation is greater or equal to the presetdeviation threshold, the function block outputs a value differentthan 1. The function determining the output of function block 222 may bea linear function mapping the deviation to the output. Also, anothertype of function may be used.

The output is then fed to a frequency adjusting function block 224 andan amplitude adjusting function block 226. Function blocks 224 and 226carry out functions mapping the output of function block 222 to an ACfrequency adjustment value P_(Adj,ω) and the reference signal adjustmentvalue P_(Adj,Amp), respectively. With the functions of function blocks224 and 226, it can be influenced, which one of the frequency of the ACvoltage system or the amplitude of the AC voltage system is to beadjusted when the DC/DC conversion stage 4 does not operate as desired.For example, when the load on the AC side is too large for the powertransfer capacities of the DC/DC conversion stage 4 and, as aconsequence, the voltage across second side terminals 12, namelyV_(DC,Sec), drops, the countermeasures of reducing the AC voltageamplitude or reducing the AC voltage frequency can be balanced. In ACvoltage systems where the DC/AC converter 2 is the only power source forthe AC voltage system, the functions of function blocks 224 and 226 maybe designed to give preference to a change in amplitude, i.e. to adjustP_(Adj,Amp) primarily. In AC voltage systems where the DC/AC converter 2is one of a plurality of power sources for the AC voltage system, thefunctions of function blocks 224 and 226 may be designed to givepreference to a change in frequency, i.e. to adjust P_(Adj,ω) primarily.

As explained above, the reference signal adjustment value P_(Adj,Amp) ismultiplied with the AC side reference signal V_(AC,Set,1) in order toyield a modified AC side reference signal V′_(AC,Set,1). The functionblock 226 ensures that P_(Adj,Amp) equals 1 when the DC/DC conversionstage 4 operates as desired. The AC frequency adjustment value P_(Adj,ω)is multiplied with the desired frequency ω_(Set) of the AC voltagesystem to yield a modified desired frequency ω′_(Set). The modifieddesired frequency w′set then serves as the basis for the waveformgenerating blocks 204, 206 _(1,WF), . . . , 206 _(N,WF). The blocks 204,206 _(1,WF), . . . , 206 _(N,WF) will set the frequencies ω, . . . , Nωto the modified desired frequency and its multiples, namely to ω′_(Set),. . . , Nω′_(Set). The function block 224 ensures that P_(Adj,ω) equals1 when the DC/DC conversion stage 4 operates as desired.

In this way, the DC/DC conversion stage 4 is helped to recuperate itsdesired operating point by the control of the DC/AC conversion stage 6.

Typical applications for the DC/AC converter 2 are isolated networks,where a DC power source, such as a combustion engine coupled to anelectric generator providing DC power, is connected to the DC side and aload network is connected to the AC side.

The voltage across the pair of second side terminals 12 of the DC/DCconversion stage 4 may be between 300 V and 800 V. The AC voltage may bebetween 100 V and 240 V. However, the DC/AC converter 2 may also besized for other applications.

It is pointed out that the circuit topology of the DC/AC converter, asdescribed with respect to FIGS. 4 and 5, may be implementedindependently from the method of controlling the DC/AC converter, asdescribed with respect to FIG. 8. In other words, the DC/AC conversionstage of the DC/AC converter of FIGS. 4 and 5 may be controlled by othermethods than the one described with respect to FIG. 8. Also, the methodof controlling the DC/AC conversion stage, as described with respect toFIG. 8, may be carried out in the context of other DC/AC converters aswell, in particular in the context of DC/AC converters with differentDC/DC conversion stage or with no DC/DC conversion stages at all.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationto the teachings of the invention without departing from the essentialscope thereof. Therefore, it is intended that the invention not belimited to the particular embodiments disclosed, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

The invention claimed is:
 1. A DC/AC converter comprising a DC/DCconversion stage with galvanic isolation and a DC/AC conversion stage,wherein the DC/DC conversion stage comprises: a pair of first sideterminals providing or receiving a first DC voltage, a pair of secondside terminals providing or receiving a second DC voltage and coupled tothe DC/AC conversion stage, at least one first side converter circuitcoupled between the pair of first side terminals, a series connection ofa plurality of second side converter circuits coupled between the pairof second side terminals, and at least one transformer circuit couplingthe plurality of second side converter circuits to the at least onefirst side converter circuit, wherein a connection point within theseries connection between two of the plurality of second side convertercircuits is coupled to the DC/AC conversion stage and forms a neutralphase point thereof.
 2. The DC/AC converter according to claim 1,wherein the series connection of the plurality of second side convertercircuits consists of an even number of second side converter circuitsand wherein said connection point is a center point of the seriesconnection of the plurality of second side converter circuits.
 3. TheDC/AC converter according to claim 1, wherein the DC/DC conversion stagehas symmetric power transfer characteristics between the pair of firstside terminals and the pair of second side terminals.
 4. The DC/ACconverter according to claim 1, wherein the number of transformercircuits is equal to the number of second side converter circuits, witheach one of the plurality of second side converter circuits beingassociated with one of the at least one first side converter circuit viacoupling through a respective transformer circuit.
 5. The DC/ACconverter according to claim 1, wherein each of the at least onetransformer circuit is configured to have a current sourcecharacteristic.
 6. The DC/AC converter according to claim 1, whereineach of the at least one transformer circuit comprises a transformer andan inductance element coupled in series with the transformer.
 7. TheDC/AC converter according to claim 1, wherein the at least one firstside converter circuit has a voltage source characteristic.
 8. The DC/ACconverter according to claim 1, wherein a first side capacitance elementis coupled between the pair of first side terminals.
 9. The DC/ACconverter according to claim 1, wherein each of the at least one firstside converter circuits comprises an H bridge circuit.
 10. The DC/ACconverter according to claim 1, wherein each of the plurality of secondside converter circuits has a voltage source characteristic.
 11. TheDC/AC converter according to claim 1, wherein each of the plurality ofsecond side converter circuits comprises a second side capacitanceelement, with the second side capacitance elements being connected inseries between the pair of second side terminals.
 12. The DC/ACconverter according to claim 1, wherein each of the plurality of secondside converter circuits comprises an H bridge circuit.
 13. The DC/ACconverter according to claim 1, wherein the even number of second sideconverter circuits is one of 2, 4, 6 and
 8. 14. The DC/AC converteraccording to claim 1, wherein the at least one first side convertercircuit is a plurality of first side converter circuits, with theplurality of first side converter circuits being connected in parallelbetween the first side terminals.
 15. The DC/AC converter according toclaim 1, wherein the number of first side converter circuits is equal tothe number of second side converter circuits.
 16. The DC/AC converteraccording to claim 1, wherein the number of second side convertercircuits is equal to the number of first side converter circuitsmultiplied by N, with N being a natural number of at least
 2. 17. TheDC/AC converter according to claim 16, wherein N second side convertercircuits, N transformer circuits and one first side converter circuitsform a respective power transfer set.
 18. The DC/AC converter accordingto claim 17, wherein the N transformer circuits of a respective powertransfer set are connected in parallel.
 19. The DC/AC converteraccording to claim 1, wherein each of the at least one first sideconverter circuits is controlled by a respective first side controlsignal and each of the plurality of the second side converter circuitsis controlled by a respective second side control signal.
 20. The DC/ACconverter according to claim 19, wherein the first side control signalsand the second side control signals are signals for controllingrespective H bridges.
 21. The DC/AC converter according to claim 19,wherein the first side control signals and second side control signalsare controlled for being pulse signals having the same periods and dutycycles, with a phase relationship between the first side control signalsand the second side control signals being controlled for controlling apower transfer between the pair of first side terminals and the pair ofsecond side terminals.
 22. The DC/AC converter according to claim 19,wherein the first side control signals and the second side controlsignals are pulse signals having a duty cycle of substantially 50%. 23.The DC/AC converter according to claim 19, wherein each second sideconverter circuit is controlled with respect to its associated firstside converter circuit, with a phase shift between the second sidecontrol signal of the respective second side converter circuit and thefirst side control signal of its associated first side converter circuitcontrolling a power transfer.
 24. The DC/AC converter according to claim1, wherein a ratio of a second side voltage of each of the second sideconverter circuits and a voltage across the pair of first side terminalsis controlled to be a transformer ratio of the respective one of the atleast one transformer circuit coupling the respective second sideconverter circuit to the at least one first side converter circuit. 25.The DC/AC converter according to claim 1, wherein each of the pluralityof second side converter circuits is controlled separately.